30 Del. C. § 542

If the Director disallows a claim for credit or refund, in whole or in part, the Director shall mail written notice of the disallowance to the taxpayer, and such notice shall set forth the reason for the disallowance. The action of the Director in disallowing all or any part of a claim for credit or refund shall become final upon the expiration of 60 days (30 days in the case of withholding taxes or retail marijuana taxes, or, in the case of other taxes imposed by Chapter 11 of this title, 120 days if the taxpayer is outside the United States) from the date on which the Director mailed the notice of disallowance to the taxpayer, unless within such period the taxpayer protests the Director’s disallowance pursuant to the provisions of § 523 of this title.